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Physics Condensed Matter Strongly Correlated Electrons

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Sunday 4 October

Strongly Correlated Electrons · Cross-listed from quant-ph

Onset of a quantum Fisher-Selke sequence in the transverse-field ANNNI model and its bilayer

Tuan-Vu Truong, Vu-Quoc-Minh Nguyen, Hoang-Long Nguyen et al. (+1 more)

We study the transverse-field axial next-nearest-neighbor Ising (ANNNI) model on the square lattice and on a bilayer. At the classical multiphase point $\kappa=J_2/J_0=1/2$, every configuration of straight domain walls that separates domains of at least two sites is a ground state. The walls are rigid, so perturbation theory in the transverse field $g=\Gamma/J_0$ can be carried out for each configuration separately, and we take it to order $g^8$. Quantum fluctuations select $\langle 3\rangle$ ($\uparrow\uparrow\uparrow\downarrow\downarrow\downarrow$) at order $g^2$, because spins next to a wall gain the most energy from virtual flips. The structures $\langle 23\rangle$, $\langle 223\rangle$ and $\langle 2223\rangle$ then open between $\langle 3\rangle$ and the antiphase $\langle 2\rangle$ at orders $g^4$, $g^6$ and $g^8$, in the order of the Fisher-Selke sequence of the classical three-dimensional model. A second layer suppresses each successive phase more strongly, beyond the stiffening of the local fields. We test the two widest phases with stochastic series expansion quantum Monte Carlo, comparing the free energies of the competing ordered states by thermodynamic integration, since annealing freezes in the modulation that forms at the ordering transition. The simulations reproduce the $\langle 3\rangle$ wedge to about 1%, and in a pre-registered test $\langle 23\rangle$ is stable at six of seven points, with the seventh resolved by post-hoc runs.
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All authors Tuan-Vu Truong, Vu-Quoc-Minh Nguyen, Hoang-Long Nguyen, Thanh-Tien Nguyen

Strongly Correlated Electrons

Floquet dressing of bulk metallic bands in 2H-NbSe2

Cong Li, Qirui Cui, Maciej Dendzik et al. (+6 more)

Floquet dressing offers a route to controlling electronic states through periodic optical driving. Its realization in solids has centred primarily on topological-insulator surface states, semiconductors and two-dimensional Dirac semimetals. Extending this control to bulk metals would enable manipulation of the itinerant states underlying metallic transport and collective order. Yet strong screening and rapid scattering have long cast doubt on its feasibility in bulk metals, while photoelectron dressing complicates its identification. Here we identify Floquet dressing of the overlapping, bulk-derived metallic bands of 2H-NbSe2 using time- and angle-resolved photoemission spectroscopy. Photon-shifted replicas in a sparse unoccupied-state window retain the parent-band dispersions and appear only during pump–probe overlap. Rotating the pump's linear polarization at fixed probe polarization redistributes replica weight between momentum-space branches. With a common parameter set, a model accounting for the optical fields inside and outside the solid captures this redistribution through coherent Floquet–Volkov interference more closely than either pathway alone or their incoherent intensity sum. These findings extend Floquet dressing to the itinerant bulk bands of a multiband metal without relying on the observation of energy gaps opened by hybridization between parent bands and their photon-dressed replicas, providing a basis for exploring how coherent optical dressing couples to metallic transport and collective electronic phases.
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All authors Cong Li, Qirui Cui, Maciej Dendzik, Magnus H. Berntsen, Wanyu Chen, Zhilin Li, Anna Delin, Wentao Zhang, Oscar Tjernberg

Strongly Correlated Electrons · Cross-listed from cond-mat.mtrl-sci

Anisotropic mesoscale magnetic scattering across the spin-reorientation transition in TbMn6Sn6

Zhuang Xu, Fan Chen, Hui Cheng et al. (+10 more)

TbMn6Sn6 is a centrosymmetric ferrimagnetic kagome metal that combines topological electronic states with strong magnetic anisotropy and a near-room-temperature spin-reorientation transition (SRT). Real-space imaging studies of thin specimens have revealed helical stripes, biskyrmion-like textures, and skyrmion-bubble-like structures in the spin-reorientation regime. Whether such local textures develop into a bulk long-range-ordered skyrmion lattice remains unresolved. Here we present small-angle neutron scattering (SANS) results probing the mesoscale magnetism of a bulk TbMn6Sn6 single crystal. We observe no sixfold SANS Bragg pattern indicative of a long-range-ordered skyrmion lattice under the explored conditions. Instead, the dominant magnetic response is an anisotropic twofold low-Q scattering component whose intensity distribution and field dependence evolve markedly across the spin-reorientation regime. In zero field, a broad and horizontally elongated low-Q signal develops above TSR and weakens as the system approaches the paramagnetic state. At 345 K with H || b, the signal is diffuse and is suppressed rapidly below about 700 Oe. By contrast, at 310 K with H || c, close to TSR, the response is more concentrated and line-shaped and persists to higher fields. The contrasts between these scans likely reflect the combined effects of rapidly evolving magnetic anisotropy near TSR and the field geometry. Our results therefore establish a bulk anisotropic mesoscale magnetic response across the spin-reorientation regime, consistent with stripe- or domain-related textures reported by real-space imaging, while not uniquely identifying their real-space morphology.
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All authors Zhuang Xu, Fan Chen, Hui Cheng, Xinmin Wang, Taisen Zuo, He Cheng, Haitao Hu, Cuiping Zhang, Qian Zhao, Michel Kenzelmann, Jonathan S. White, Junying Shen, Xin Tong

Strongly Correlated Electrons · Cross-listed from cond-mat.supr-con

Superconducting landscape of the strongly correlated bilayer Hubbard model

Motoharu Kitatani, Yusuke Nomura, Shiro Sakai et al. (+1 more)

Coupling two strongly correlated layers opens a distinct route to repulsion-driven superconductivity. Yet the same interlayer coupling makes the optimization of superconductivity highly nontrivial. Using the dynamical vertex approximation, we determine the superconducting landscape in the enlarged parameter space spanned by filling and interlayer hopping. We identify an optimal regime at intermediate interlayer hopping and moderate doping away from half filling, where the system exhibits high superconducting transition temperatures ($T_{\rm c}$) with an interband $s_{\pm}$-wave gap structure. Toward half filling, however, strong self-energy effects can suppress the superconducting tendency by reducing the low-energy spectral weight. We further show that a Fermi-surface distortion induced by longer-range hoppings can weaken this suppression and enhance $T_{\rm c}$. Our results establish how filling, interlayer hopping, and Fermi-surface geometry jointly optimize the superconducting regime in the strongly correlated bilayer Hubbard model.
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All authors Motoharu Kitatani, Yusuke Nomura, Shiro Sakai, Ryotaro Arita

Saturday 3 October

Strongly Correlated Electrons · Cross-listed from cond-mat.other

$\mathcal{PT}$-symmetry as Effective Time Reversal Symmetry for Anderson Localization in a Collinear Antiferromagnet

Kim-Khuong Huynh, Takuma Ogasawara, Motoi Kimata et al. (+7 more)

We show that $\mathcal{PT}$-symmetry acts as the effective time-reversal symmetry governing Anderson localization in the antiferromagnet BaMn$_2$Bi$_2$, whereas the general time-reversal symmetry is broken by its long-range magnetic order. The magnetoconductance follows the laws established from breaking time-reversal symmetry throuhg the orbital coupling of charge carriers to the magnet vector potential, however, the degrees of symmetry breaking are qualitatively modulated by the magnetocrystalline anisotropy stabilizing $\mathcal{PT}$-related sublattices. The quantum interferences and hoping amplitudes governing the phenomena are drastically impaired by transverse magnetic fields, which readily cant $\mathcal{PT}$-related magnetic sublattices. Accordingly, when the $\mathcal{PT}$-related texture is weakly perturbed by longitudinal fields orienting along the sublattices, the localization is minutely affected. The robustness of the $\mathcal{PT}$-enforced degeneracies is thus governed by exchange and magnetocrystalline interactions, in contrast to the exact $\mathcal{PT}$-symmetry of the full Hamiltonian, which is broken by any finite field.
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All authors Kim-Khuong Huynh, Takuma Ogasawara, Motoi Kimata, Sofie Søby Leiszner, Nhu-Quynh Thi Phan, Michael Anthony Quintero, Frej Søren Rattenborg, Denis Arčon, Bo Brummerstedt Iversen, Katsumi Tanigaki

Strongly Correlated Electrons

Peeling off Abelian Anyons and Proliferation Transitions

Meng Cheng and Nathan Seiberg

We study the invertible one-form global symmetry of a (2+1)-dimensional topological quantum field theory (TQFT). A universal description of this symmetry “peels off” all the Abelian anyons from the rest of the TQFT. This allows us to study this symmetry and some of its consequences, independently of the details of the rest of the system. We then describe these Abelian anyons using $\mathrm{U}(1)$ gauge fields with Chern-Simons terms and couple them to a scalar field, thus constructing a particular non-topological theory. This new theory, our transition field theory, exhibits a phase transition from the original TQFT to another phase. The latter is either gapless or another TQFT. This discussion generalizes and extends our earlier work (arXiv:2603.00245). In particular, we give a more detailed description of the global symmetries and their fates in the various phases.
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Strongly Correlated Electrons

Does a pole in the self-energy imply a violation of the Luttinger theorem?

Artem Abanov and Andrey Chubukov

Luttinger theorem relates Fermi surface volume to electron density per spin $n$. It holds in an arbitrary Fermi liquid but is widely believed to break when a fermionic self-energy has a pole. We show that this is not necessarily the case. As an example, we consider a fermionic system consisting of two gapped bands, originating from a self-energy $\Sigma (\omega, \mathbf{k})$ with a pole, and a narrow residual quasiparticle band crossing $\omega =0$, with a small residue $Z \ll 1$ (a residual Fermi liquid). We consider Model I, in which $Z$ also renormalizes quasiparticle bandwidth $W$ to $ZW$ and Model II, in which $W$ remains unrenormalized. The Green's function for such a system $G(\omega, \mathbf{k})$ contains two zeros and three poles — one from the quasiparticle band and two from the gapped bands. Yet, we show that in Model I, the Luttinger-Ward analysis is fully applicable for any $Z >0$, and the results are exactly the same as if the gapped bands did not exist. For Model II, the Luttinger-Ward analysis recovers once $Z$ exceeds some critical value, which depends on fermionic density.
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Strongly Correlated Electrons

Determining the j-Specific Unoccupied 5f Electronic Structure in Actinides

J. G. Tobin

An empirical comparison of high resolution M4,5 X-ray Absorption Spectroscopy (XAS), based upon High Energy Resolution Fluorescence Detection (HERFD), with Bremsstrahlung Isochromat Spectroscopy (BIS), the high energy variant of Inverse Photoelectron Spectroscopy (IPES), of various actinide materials is presented here. It will be shown that XAS-HERFD is essentially the j-specific version of BIS, providing a direct measure of the separate 5f5/2 and 5f7/2 Unoccupied Density of States (UDOS).
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Strongly Correlated Electrons

State-selective entanglement and point-gap topology in the unidirectional Bose-Hubbard chain

Chuan-Fu Lin, Yu-Chin Tzeng and Po-Yao Chang

We study the effect of repulsive interactions on skin modes and entanglement in the unidirectional Bose–Hubbard model. Although the ground state of this integrable model undergoes a superfluid–Mott transition with suppressed skin accumulation, the corresponding change in entanglement depends on the selected many-body state. We compare the ground state with the steady state, defined as the largest-imaginary-energy eigenstate in the zero-momentum sector. For sixteen sites at unit filling, we find that the ground-state density approaches uniform filling and its entanglement decreases toward the Mott limit, whereas the steady states exhibit a nonmonotonic entanglement crossover. At intermediate interaction ($U/t=10$), the second Rényi entropy of the steady state develops four spatial maxima. We show that these maxima are accounted for by competing subsystem-number sectors within a dominant configuration manifold of four doublons and four holons, but not by the subsystem particle-number variance alone. The dominant global onsite-pair manifold retains the same entropy peak structure. At strong interaction, projection onto the one-doublon–one-holon manifold closely reproduces the full entropy profile and its two edge peaks. Furthermore, we demonstrate that the many-body winding about the origin remains nonzero at strong interaction; an exact two-site example shows how it becomes ill-defined in the singular projected $U/t\to\infty$ limit. Our results distinguish ground-state skin suppression from state-selective entanglement and point-gap topology.
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Strongly Correlated Electrons

Magneto-transport and electronic structure studies of ternary antimonides, La$T$Sb$_2$ ($T$ = Cu, Ag)

Himanshu Pant, Neeraj Bhatt, Deepali Sharma et al. (+7 more)

We investigate Sb square net-based Dirac semimetallic ternary antimonides La$T$Sb$_2$ ($T$ = Cu, Ag) using magnetotransport and photoelectron spectroscopic studies, complemented by density functional theory (DFT) based ab initio calculations. Our results show that both compounds exhibit a positive temperature coefficient of resistivity, linear magnetoresistance (MR), and a deviation from Fermi liquid behaviour at low temperatures (2-30 K). But only $T$ = Ag compound exhibit signature of charge density wave (CDW) phenomena and the results of its temperature dependent MR data suggest that single-band semiclassical transport is not sufficient to understand the transport behaviour. $\text{DFT}+U$ studies show that in both samples, an on-site Coulomb interaction ($U$) for Cu 3$d$ and Ag 4$d$ electrons, in the presence of spin-orbit coupling (SOC) is required to reproduce experimental valence band spectra. Sb 3$d$, 4$d$ core-level spectra combined with $\text{DFT}+U$ calculations and La 3$d$ spectra confirm the presence and effects of inequivalent Sb sites for both the samples. In contrast to the $T$ = Cu compound, which follows expected temperature-dependent core-level sharpening, the $T$ = Ag compound exhibits an anomalous low-temperature broadening in the CDW phase. The analysis of Sb 3$d$ spectra suggests Sb square nets are linked to CDW phenomena in the $T$ = Ag compound. Temperature-dependent ultraviolet photoemission spectroscopic studies on $T$ = Ag show a small decrement in spectral DOS at the $\epsilon_F$ for the temperature below the CDW transition temperature. These core-level studies provide a valuable spectroscopic benchmark for comparing CDW and non-CDW systems and clarifying the role of local electronic environments in CDW formation.
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All authors Himanshu Pant, Neeraj Bhatt, Deepali Sharma, Asif Ali, A. Thamizhavel, Saurabh Singh, Kentaro Kuga, Tsunehiro Takeuchi, Ravi Shankar Singh, R. Bindu

Strongly Correlated Electrons · Cross-listed from quant-ph

How to distinguish chaos from integrability using OTOC

Alexey Milekhin and Anders Wallberg

The decay of out-of-time-ordered correlators (OTOCs) to a small residual value signals the scrambling of quantum information. This behavior is generic: it occurs in both chaotic and interacting integrable many-body systems, and therefore cannot by itself distinguish between them. In this paper we analytically study the late-time saturation value of a closely related observable, operator purity, and show that its scaling with system size robustly distinguishes chaotic from integrable systems. Our approach is based on the Mazur bound, which provides concrete scaling estimates and makes explicit why the locality of integrable charges matters. We support our findings with numerical simulations of one-dimensional spin chains.
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Friday 2 October

Strongly Correlated Electrons

Disorder into Correlation: A Microscopic Mechanism for Sub-McIntyre Avalanche Noise

Shafat Shahnewaz, Hannaneh Karimi, Joe C. Campbell et al. (+1 more)

Avalanche multiplication is intrinsically stochastic, yet experiments reveal excess noise below the minimum predicted by McIntyre's local-field theory. The microscopic origin of this noise suppression remains incompletely understood. We show that polar-optical-phonon emission, confined to a narrow forward cone, competes with elastic alloy scattering that diffuses field-collimated hot carriers out of the cone. Suppressed phonon cooling narrows the ionization path beyond the dead space and synchronizes successive ionizations into a dynamic staircase. This explains the low noise of Sb-based alloys and HgCdTe and makes alloy disorder a design parameter.
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All authors Shafat Shahnewaz, Hannaneh Karimi, Joe C. Campbell, Avik W. Ghosh

Strongly Correlated Electrons

Correlated Low-Energy Model of Monolayer 1H-NbS$_2$: A cRPA+DMFT Study

Karollaine C. Leite, Jorge L. B. de Faria, Luiz Antonio Ribeiro Junior et al. (+3 more)

Monolayer $1\mathrm{H}\text{-}\mathrm{NbS}_2$ hosts a well-separated metallic band of predominant Nb-$d_{z^2}$ character, providing an ideal platform to probe electronic correlations within a single-band setting. In this work, we present a comprehensive study of $1\mathrm{H}\text{-}\mathrm{NbS}_2$ by combining first-principles Wannier interpolation, constrained random-phase approximation (cRPA), and DFT+DMFT. We show that an effective single-band model accurately captures the low-energy electronic structure, reproducing key experimental features from ARPES and STS $dI/dV$ spectra, including a characteristic Van Hove singularity. While static DFT+$U$ fails to describe the correlated metallic nature of the system, DFT+DMFT successfully accounts for the spectral weight redistribution and dynamic correlations. By defining the cRPA target subspace directly within the isolated Nb-$d_{z^2}$ band, we obtain a renormalized local interaction of $U_{\mathrm{cRPA}} = 1.138~\mathrm{eV}$. This interaction stabilizes a strongly correlated metallic state that remains robust upon cooling. Remarkably, this minimal single-band model captures a spectral broadening similar to that previously reported in more elaborate treatments - incorporating larger local interactions ($U \sim 1.8~\mathrm{eV}$, derived from a multiorbital cRPA construction), intersite Coulomb terms ($V$), and electron-phonon coupling - demonstrating that a consistently downfolded local interaction alone can already capture key aspects of the experimentally observed low-energy behavior of monolayer $1\mathrm{H}\text{-}\mathrm{NbS}_2$.
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All authors Karollaine C. Leite, Jorge L. B. de Faria, Luiz Antonio Ribeiro Junior, Alexandre C. Dias, Helena Bragança, Alberto S. de Arruda

Strongly Correlated Electrons

ToF-PAX-RIXS: A Time-of-Flight Spectrometer for combined ARPES and RIXS

Chafic Fawaz, Tom Lacmannn, Yaryna Lytvynenko et al. (+24 more)

We present ToF-PAX-RIXS, a compact instrument that combines time-of-flight momentum-microscopy angle-resolved photoemission spectroscopy (ARPES) with resonant inelastic X-ray scattering (RIXS). The latter is based on photoelectron spectrometry for analysis of X-rays (PAX). The two techniques are implemented with a single multimode electron-optical column, allowing measurements to be performed sequentially on the same sample under identical experimental conditions. In ARPES mode, the instrument records the photoemission intensity as a function of kinetic energy and two in-plane momentum components. In PAX-RIXS mode, scattered soft X-ray photons are absorbed in a thin metallic converter, and the generated photoelectrons are analyzed in energy and position of emission on the converter. The electron kinetic energy retains the spectral information of the scattered photons, while the position is related to the scattering angle and therefore to the momentum transfer, enabling simultaneous access to a finite momentum-transfer interval. We describe the mechanical design, converter assembly, scattering geometry, and multimode electron optics of the instrument. Its PAX-RIXS performance is benchmarked at the Cu $L_3$ edge using CaCuO$_2$, with Au, Ag, and Pt converters. The measured spectra reproduce the principal features obtained with a conventional grating RIXS spectrometer, and deconvolution yields an effective reconstructed energy resolution better than $120$~meV under the present conditions. Finally, combined ARPES and PAX-RIXS measurements on the same Ca-doped YBa$_2$Cu$_3$O$_{6.45}$ sample demonstrate the capability of the instrument to correlate electronic structure and collective excitations within a common compact platform.
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All authors Chafic Fawaz, Tom Lacmannn, Yaryna Lytvynenko, Sergii Chernov, Olena Tkach, Siegmar Roth, Jan O. Schunck, Amon Lanz, Jakob Dilling, Lukas Bruckmeier, Markus Scholz, Frank Scholz, Arne Meyer, Dirk Fuchs, Lara Schmieder, Yiran Liu, Matteo Minola, Bernhard Keimer, Dmytro Kutnyakhov, Kai Rossnagel, Olena Fedchenko, Angela Wittmann, Moritz Hoesch, Gerd Schönhense, Matthieu Le Tacon, Hans-Joachim Elmers, Martin Beye

Strongly Correlated Electrons · Cross-listed from cond-mat.mes-hall

Escaping the Composite Fermi Sea: An Incompressible State at Half Filling in Graphene's Lowest Landau Level

Domagoj Perkovic, Ziwei Wang and Steven Herbert Simon

Recent experiments in monolayer and trilayer graphene have observed incompressible even-denominator fractional quantum Hall states within narrow regions of parameter space associated with crossings of zeroth Landau levels. This is surprising because at half filling the Coulomb interaction in the zeroth Landau level is expected to favour a compressible composite Fermi liquid. Here we provide the first quantitative energetic study of the candidate incompressible phases and of the microscopic mechanisms that can stabilise them near such Landau-level crossings. Using exact diagonalization and infinite density-matrix renormalisation group, we first study ultra-short-range corrections to the Coulomb interaction arising from finite layer separation, lattice-scale effects, and electron-phonon coupling. We find that a sufficiently strong attractive short-range interaction drives the composite Fermi liquid first into a weakly paired \(d\)-wave pseudospin-singlet state and subsequently into a strongly paired state. However, microscopic estimates of these short-range corrections are far too small to stabilise either incompressible phase. We then incorporate Landau-level mixing through screening within the random-phase approximation. Screening substantially reshapes the effective interaction, suppresses the weak-pairing regime, and strongly enhances the tendency toward strong pairing. For a realistic range of microscopic parameters, the strongly paired phase is stabilised over a substantial fraction of that range. Our results identify the interplay of Landau-level mixing and ultra-short-range interactions as a plausible microscopic mechanism for the experimentally observed incompressible states in monolayer and trilayer graphene and point toward a strongly paired composite-fermion phase as their candidate ground state.
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Strongly Correlated Electrons · Cross-listed from quant-ph

Magic Quantum Code Surgery

Kathleen Chang, Anasuya Lyons, Yuanjie Ren et al. (+5 more)

Non-Clifford operations required for universal quantum computation can be implemented by measuring logical Clifford operators. We present a generalized framework to fault-tolerantly measure transversal Clifford logical operators in quantum low-density parity check (QLDPC) codes. Our protocol takes any QLDPC code and deforms the code across additional ancillary qubits such that a product of stabilizers measures the desired logical Clifford operator. We prove that the deformed, Clifford-stabilized code remains LDPC and that its distance is preserved up to a constant factor independent of the number of physical qubits. We further prove that the Clifford measurement has a fault distance against data qubit and measurement errors that grows linearly with the distance of the original code. We apply our methods to implement logical non-Clifford gates in algorithmically relevant subroutines and to prepare magic states in quantum stabilizer codes.
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All authors Kathleen Chang, Anasuya Lyons, Yuanjie Ren, Harald Putterman, Nathanan Tantivasadakarn, Victor V. Albert, Benjamin J. Brown, Dominic J. Williamson

Strongly Correlated Electrons · Cross-listed from quant-ph

Exact Temporal Criticalities of Entanglement and Negativity in Gaussian and Non-Gaussian Dissipative Kitaev Chains

Shrey Chowdhary, Sanket Chirame, Stephan Haas et al. (+1 more)

When the two largest eigenvalues of an entanglement or negativity spectrum meet, the gap of the corresponding entanglement or negativity Hamiltonian closes while entropies stay smooth, an event we call a temporal criticality. We ask when such criticalities occur in open fermionic systems, in which order, and whether they precede the geometric decoherence time, at which the negativity starts to fall. We compare loss and gain, which respect fermion parity only on average (weak symmetry) and keep the state Gaussian, with density dephasing, which respects it in every jump (strong symmetry) and makes the state non-Gaussian. We find that Gaussianity, rather than the symmetry, fixes the criterion for a criticality. In a Gaussian state the entanglement gap of a subsystem closes exactly when its two parity sectors carry equal weight, and the negativity gap closes exactly when this happens for the subsystem or its complement, so the negativity criticality never comes after the entanglement one. Uniform balanced loss and gain only shrink the two parities and leave both criticalities in place at every rate, for any quadratic Hamiltonian. In an open Kitaev chain, weak imbalanced loss and gain shift the criticalities by amounts set by the occupations at the cut, and at strong rates they allow a closing near the relaxation time only when an odd number of neighboring sites pass half filling. For dephasing we solve the non-Gaussian dynamics exactly at the point where the bond terms of the chain commute. Weak dephasing delays both criticalities and, away from the chain ends, moves them ahead of the parity balance, where the parity rules of Gaussian states would place them. As the dephasing rate approaches critical damping, both criticalities recede to infinite time, and at strong dephasing the negativity starts to fall long before either gap begins to shrink.
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All authors Shrey Chowdhary, Sanket Chirame, Stephan Haas, Rishabh Jha

Strongly Correlated Electrons

The ALPS project release 3.0: open source software for strongly correlated systems

F. Alet, T. Chen, A. Feiguin et al. (+16 more)

We present release 3.0 of the ALPS (Algorithms and Libraries for Physics Simulations) project, an open-source software project to develop libraries and application programs for the simulation of strongly correlated quantum lattice models such as quantum magnets, lattice bosons, and strongly correlated fermion systems. As in previous releases, development is centered on common data formats, on libraries to simplify and speed up code development, and on full-featured simulation programs that let non-experts carry out serial or parallel numerical simulations using the important algorithms for quantum lattice models: classical and quantum Monte Carlo (QMC) using non-local updates, extended-ensemble simulations, exact and full diagonalization (ED), the density matrix renormalization group (DMRG), and continuous-time QMC solvers for dynamical mean-field theory (DMFT). Major changes in release 3.0 include distribution of the pyalps binary through the Python Package Index (pip install pyalps) and through Spack for HPC systems; migration of development to GitHub with continuous integration and automated testing; relicensing of the package under the permissive MIT license; a rebuilt documentation and tutorial website, including a set of Jupyter-notebook tutorials and localized content; a broad modernization of the C++ codebase (C++17 compliance, Boost and NumPy 2.0 compatibility, and warning and dead-code cleanup) together with a major DMRG update and associated reliability and build-compatibility fixes; the removal of legacy components (the VisTrails provenance integration and the TEBD, MPS, and directed-worm-algorithm application codes); archival of release 3.0.0 with a Zenodo DOI; and a formal governance and sustainability model developed under the US National Science Foundation (NSF) POSE program. The software is available at https://alps.comp-phys.org.
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All authors F. Alet, T. Chen, A. Feiguin, E. Gull, S. Iskakov, J. P. F. LeBlanc, F. Lin, A. Mirmira, G. Möller, L. Pollet, M. Rosales, V. W. Scarola, H. Shinaoka, H. Terletska, S. Todo, M. Troyer, M. Wallerberger, P. Werner, T. M. R. Wolf

Strongly Correlated Electrons · Cross-listed from math-ph

Time-dependent integrable quantum field theories with dynamical boundaries

Pradip Kattel

We present a method for constructing time-dependent integrable chiral quantum field theories on a finite spatial interval from the scattering and reflection data of an autonomous integrable model. Each particle carries a spectral label that is constant along its free trajectory. Requiring a right–left scattering event at a fixed time to be independent of its position forces the map from characteristic coordinates to spectral labels to be affine, with the same slope for both chiralities. As a result, all bulk scattering events at a given time sample one common spectral argument, and the two boundary reflections sample the corresponding endpoint arguments. The bulk and boundary drives are not independent time-dependent deformations: their time dependence is fixed by the same affine spectral coordinate. Successive reflections at the two endpoints translate the spectral label by a finite amount. Following a particle through a complete round trip then produces a difference transport whose consistency gives the boundary quantum Knizhnik–Zamolodchikov equations. The Yang–Baxter and reflection equations ensure local compatibility, while BqKZ flatness ensures compatibility of complete round trips. We then reconstruct the local Hamiltonian by inverting the one- and two-particle matching problems. The bulk matrix fixes the contact interaction, while the reflection matrix fixes the corresponding boundary coupling. The same autonomous factorized data determine the boundary qKZ transport problem and the local driven Hamiltonian. Once the boundary qKZ system is solved, its characteristic pullback gives the corresponding time-dependent many-body wavefunction. We illustrate the construction with rational $SU(N)$ models and an $O(3)$-invariant model with dynamical boundary impurities, obtaining driven Kondo interactions whose couplings are fixed by the same affine spectral coordinate.
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Strongly Correlated Electrons · Cross-listed from quant-ph

Realizing Braided Fusion 2-Categories as (3+1)D Mixed-State Topological Orders

Ryohei Kobayashi, Abhinav Prem and Matthew Yu

We construct large families of mixed-state topological orders in (3+1)D, including intrinsically mixed-state topological orders. We realize these theories by relating the data parameterizing braided fusion 2-categories to the strong higher-form symmetries and their anomalies that characterize mixed-state topological order. We focus here on bosonic theories, which include theories which admit an emergent fermion in the spectrum of line operators. Our general framework is exemplified on the lattice, where we present concrete models and identify how the data of braided fusion 2-categories are physically encoded. We then formalize this data and show that every braided fusion 2-category arises from a general construction (decoherence followed by strong and weak gauging) as the category of topological operators of a (3+1)D mixed state, with intrinsically mixed-state topological orders distinguished by the presence of strong higher-form symmetries whose anomalies cannot be realized in any (3+1)D gapped ground state. Notably, we show that (3+1)D admits a new mechanism with no (2+1)D counterpart: intrinsically mixed-state order whose topological operators braid non-degenerately, but which carries a gravitational anomaly.
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Strongly Correlated Electrons · Cross-listed from quant-ph

Maximum-Entropy Extension of Quantum Correlation Functions from Short Real-Time Dynamics

Filippo Maria Gambetta, Sabrina Yue Wang and Raul A. Santos

Correlation functions are central to understanding the behavior of quantum systems, with their spectra giving direct access to quasiparticle and collective excitations. However, the long-time evolution required for sufficient spectral resolution is challenging to reach for both classical and quantum simulations. In this work, we introduce a principled approach for extending time series of quantum correlation functions and reconstructing their spectral densities, based on their connection with covariance sequences of stationary stochastic processes. This connection enables us to apply Burg's maximum-entropy principle to find the least-committal extension compatible with the measured values of the correlation function, which corresponds to an all-pole autoregressive model whose coefficients solve a set of Yule–Walker equations. The resulting extension is positive semi-definite by construction, guaranteeing the positivity of the spectrum and the automatic satisfaction of its total spectral weight sum rule. The approach applies to both scalar and matrix-valued correlation functions, including those with highly structured or continuous spectra. The generalization to the matrix case makes it possible to trade simulation time for additional measured correlation functions, enabling accurate time-series extension and spectral reconstruction from short-time data, even in the presence of noise.
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Strongly Correlated Electrons · Cross-listed from quant-ph

Efficient Block Encoding of Structured Hamiltonians by Separating Where and What

Alessandro Summer and François Jamet

In fault-tolerant block encodings of structured Hamiltonians, the support geometry can set how the non-Clifford cost of SELECT scales with the system size, however many operators act on each support. We construct permute-act-unpermute circuits: a CSWAP network fixed by this geometry brings a selected support to a fixed target register, a shared local circuit acts there, and the permutation is undone. The operators add only an $XZ$ mask on this register, at a cost independent of the system size, and one query can mix several support geometries. For two-site supports, we establish and attain the minimum CSWAP count within address-controlled networks of site transpositions. Compiled for Pauli terms, these circuits use $8\mathrm{S}N+o(N)$ $T$ gates with $O(\log N)$ permutation work qubits, where $N$ is the number of system qubits and $\mathrm{S}$ is determined by the support geometry, with $\mathrm{S}=1$ for nearest-neighbour interactions and $\mathrm{S}=3/2$ for all-to-all pairs; in both cases the correction is $O(\log N)$. We also introduce a bridged CSWAP, which remembers the logical value of its temporary AND across the target action and repairs it from the prepared labels, so that the inverse CSWAP undoes the exchange without recomputing the value. When the repair is Clifford, it halves the $T$ count of a matched forward-inverse CSWAP pair at the cost of one retained work qubit. For a Heisenberg ring, the complete block-encoding query reduces the $T$ count by a factor approaching $3$ as the system size grows, relative to flat and product-wise Pauli walks. For a five-orbital Anderson impurity model, which combines nearest-neighbour and all-to-all support geometries, the reduction is about $1.7$ at large bath sizes, relative to the cheapest of three comparators, each built from the methods of the references it follows.
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Strongly Correlated Electrons

Finite-momentum pairing and magnetic halos in a spin-imbalanced Holstein model

Chunhan Feng, George Batrouni and Richard Scalettar

Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) superconductivity, Cooper pairing at finite-momentum driven by spin imbalance, remains difficult to study numerically in two dimensions. The fermion sign problem in the spin-imbalanced attractive Hubbard model severely limits quantum Monte Carlo access to the low temperatures where pairing correlations are most pronounced. We show that the Holstein model, in which the effective attraction is retarded by phonon dynamics, strongly suppresses sign fluctuations under spin imbalance, enabling unbiased determinant quantum Monte Carlo simulations deep in this regime. With increasing magnetization, zero-momentum pairing evolves into finite-momentum FFLO pairing, evidenced by oscillating real-space pair correlations and a shift of the pairing susceptibility peak to finite $\vec{Q}$. The finite-momentum susceptibility substantially exceeds its zero-momentum value. At high density $n \sim 0.875$, near magnetization $m \approx 1-n$ where the majority-spin density approaches half filling, FFLO pairing coexists with checkerboard charge-density-wave order at short range, while staggered longitudinal spin correlations emerge as a magnetic halo at larger distances. These spatial textures provide experimentally accessible signatures of finite-momentum pairing and motivate cold-atom realizations of FFLO physics.
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Strongly Correlated Electrons · Cross-listed from quant-ph

Exact Recovery for Non-Abelian Surface Codes

Alison Warman, Nathanan Tantivasadakarn and Sakura Schafer-Nameki

We study exact recovery for non-Abelian topological surface codes based on the quantum double $D(G)$ of any finite group $G$. We determine an orthogonal error basis, that is comprised of group multiplication and irreducible representation operators. Whenever the errors are supported on closed loops on the lattice or dual lattice, there is a redundancy in the basis, due to stabilizers. To remedy this, we introduce a gauge-fixing that results in a complete and orthogonal error basis on the code space. Assuming as input predetermined, neutral correctable error clusters, we construct charge and flux transfer circuits that move the errors onto ancillas that then get projected out. This is an exact and deterministic recovery protocol, applicable to surface codes for any finite, in particular non-Abelian, group $G$.
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Strongly Correlated Electrons

Decohering Kitaev's Sixteenfold Way: A Holographic Approach

Shuyu Zhang, Linhao Li, Zhen Bi et al. (+2 more)

We study anyon decoherence in 2D topological order, with the fermion decoherence in Kitaev's sixteenfold way as explicit examples. The sixteenfold way includes chiral topological phases, which are believed to admit no commuting projector Hamiltonian, making it challenging to study their decoherence directly. We further conjecture the density matrix produced by this decoherence induced phase transition can be obtained by a three-step procedure: first lift the topological order to its Walker-Wang model, followed by a transition to another Walker-Wang model, and finally trace over the bulk degrees of freedom. This establishes a connection between the decoherence of a topological order and a pure state phase transition in its 3D counterpart. Many examples are given. In particular, we show that decohering the fermion in each of the sixteen states always yields an intrinsically mixed-state topological order, a consequence of the fact the corresponding 3D bulk always transits into the 3D fermionic toric code (fTC). Our result suggests a systematic method for studying anyon decoherence in chiral topological orders at the level of commuting projector Hamiltonians.
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All authors Shuyu Zhang, Linhao Li, Zhen Bi, Tzu-Chieh Wei, Zijian Song

Strongly Correlated Electrons

Consistent perturbation expansions in screened interactions

Abdallah Arthur El Sahili, Francesco Sottile and Lucia Reining

Many-body perturbation theory is a powerful framework to calculate spectroscopic quantities, as well as the total energy. However, the field suffers from the fact that different flavors of approximations lead to a wide range of results. We present several in principle exact expansions and discuss how to use them consistently. We concentrate on the first two orders in the respective screened Coulomb interaction. Of special interest is a consistent expansion around the Kohn-Sham system. We illustrate the performance of the various choices using the symmetric Hubbard dimer over a wide range of correlation strength. In particular, the expansion based on exact Kohn-Sham ingredients stands out as a promising route towards good energies and good spectra.
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Strongly Correlated Electrons

Melting phase diagram of the two-dimensional electron solid in a perpendicular magnetic field

M. Yu. Melnikov, D. G. Smirnov, A. A. Shashkin et al. (+3 more)

We determine the thermal melting phase diagram of the electron solid by measuring the onset density of the double-threshold voltage-current curves in the insulating state in the ultra-high mobility two-dimensional electron system in SiGe/Si/SiGe quantum wells at different temperatures in perpendicular magnetic fields. The introduction of a magnetic field leads to a considerable increase in the onset density, which is strongest at the lowest temperatures and decreases with increasing temperature, and the melting temperature increases with magnetic field. These data are in agreement with theoretical predictions. Unexpectedly, the melting temperature as a function of electron density in a fixed magnetic field reveals a plateau, which can be interpreted in terms of resonance modes in the crystallites.
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All authors M. Yu. Melnikov, D. G. Smirnov, A. A. Shashkin, S.-H. Huang, C.~W. Liu, S. V. Kravchenko

Strongly Correlated Electrons · Cross-listed from nucl-th

Ab initio Green's function theory of superfluid neutron matter

F. Marino, W. G. Jiang, C. Barbieri et al. (+1 more)

At the low densities encountered in the inner crust of neutron stars, neutron matter becomes superfluid, directly influencing macroscopic phenomena such as star cooling. However, a precise quantitative understanding of this system has been hindered by uncontrolled many-body approximations. In this letter, we exploit the nonperturbative Gorkov algebraic diagrammatic construction method to provide a unified ab initio description of the equation of state and the single-particle spectral functions of homogeneous neutron matter, spanning from the dilute regime to saturation density. We identify clear microscopic signatures of superfluidity in the excitation spectrum and quantify the pairing gaps associated with realistic chiral Hamiltonians, estimating for the first time their sensitivity to the many-body truncation. Our robust handling of both dynamical and pairing correlations paves the way for exploring electroweak processes across the regimes relevant to neutron stars.
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All authors F. Marino, W. G. Jiang, C. Barbieri, G. Colò

Strongly Correlated Electrons · Cross-listed from quant-ph

Efficient Sampling for Many-Body Fermionic Non-Gaussianity

Ryota Matsuda, Masahiro Hoshino and Yuto Ashida

Fermionic non-Gaussianity is a key resource for universal quantum computation, and its behavior in quantum many-body systems has attracted growing interest. Recently, a convolution-based measure, which we call the magic Rényi entropy (MRE), has been proposed as a resource measure of non-Gaussianity, but its evaluation is limited to small systems due to the computational cost growing exponentially with system size. In this work, we develop a perfect-sampling method to calculate the second-order MRE from matrix product states (MPSs). Our method uses a bounded estimator to control sampling fluctuations and a recursive sweep to draw the samples directly from the input MPS, which allows us to generate each sample with $\mathcal O(nD^3)$ time, where $n$ and $D$ denote the number of modes and MPS bond dimension, respectively. We benchmark our method on the XXZ chain with up to $128$ sites, demonstrating that the MRE captures intricate higher-order correlations at large scales, which are inaccessible to covariance-based measures. These results provide a computational tool that quantitatively evaluates many-body fermionic non-Gaussianity by accounting for higher-order correlations.
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Strongly Correlated Electrons · Cross-listed from physics.chem-ph

Bogoliubov self-consistent $GW$ and second-order Green's function methods for attractive fermionic interactions

Mauricio Rodríguez-Mayorga, Johannes Tölle and Pierre-François Loos

Describing fermionic systems with attractive interactions, such as superconductors and atomic nuclei, beyond the mean-field level remains a central challenge. In this work, we investigate the performance of Bogoliubov fully self-consistent $GW$ (Bog.~sc$GW$) and second-order Green's function (Bog.~scGF2) methods, implemented within a unified imaginary-time and imaginary-frequency framework. Using the attractive Hubbard model as a benchmark against full configuration interaction (FCI), we find that Bog. scGF2 provides a more accurate description than Bog.~sc$GW$. In particular, Bog.~sc$GW$ systematically overestimates the correlation energy, with errors increasing rapidly as the attractive interaction strength grows. These results highlight the importance of exchange contributions in strongly correlated fermionic systems, even when exchange effects are included only at low order in perturbation theory.
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